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Tanks Internal To Avoid Short Circuiting Of Oily Water

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#1 Arham


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Posted 18 April 2015 - 01:56 PM

Dear all,


I am sizing a disposal water tank with 471 m3 of geometric capacity. Tank is blanketed with N2.

Water inlet flow rate to the tank is 150 m3/hr with 400 ppmv of oil content. Oil will float on surface of liquid and removed through skimmer.


Tank detail is as under;


Dia = 10 m

Length = 6 m


HHLL : 5700 mm

HLL: 5500 mm

LLL: 2000 mm

LLLL: 1850 mm


Liquid inlet nozzle is at 1600 mm, 

pump outlet nozzle is at 1000 mm.



I have two questions which are as under;


1- For oil water separation :


I have find out terminal (Rising velocity for 60 micron oil droplet) through stokes law and calculated rising time of droplet from bottom inlet nozzle to HHLL and compared with residence time of disposal water. 


As per my understanding, if particle rising time is less than residence time this means separation is okay.

Is this understanding is correct ?


Also i have provided inlet distributor at inlet nozzle for better distribution of oily water inside tank. is this understanding is correct ?


2- How to avoid Short circuiting:


To avoid short circuiting inside tank, client is asking for concentric pipe at pump outlet nozzle. How this concentric pipe works ?


If this concentric external pipe top is open above HHLL and internal pipe top is upto HLL then how water will rise from bottom of concentric pipe through annular area upto internal pipe height as N2 pressure is acting on open concentric pipe?


if concentric pipe is closed from the top end then siphon will create that will rise water from bottom to top of internal pipe through annular area of pipe and then pump will suck out through internal pipe. Here another problem will occur. if water level will start decreasing (say slightly above LLL), then there might be a chance that oil floating at surface will pass through concentric pipe due to siphon along with water?



Plz guide me in this regards.





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